1. Field of the Invention
The present invention relates to a rechargeable cell that employs manganese dioxide as a cathode material, zinc as an anode material and aqueous zinc sulfate solution as an electrolyte. More specifically, it relates to a rechargeable cell, characterized in that an appropriate amount of manganese (II) salt and carbon powder are added in order to inhibit the decrease in cell capacity resulting from repeated charge/discharge cycling.
2. Description of the Prior Art
Cells are widely used as a power source for electronic and electric apparatuses. The recent trends towards miniaturization, lightweight, higher performance, portability, and personal use of electronic and electric apparatuses have brought about increased demand for rechargeable cells that are economical and suitable for long term use. The necessity of the development and production of rechargeable cells stems from the need to replace the demand for primary cells, which are once discharged and discarded, and to cut down serious energy loss and environmental problems. In view of the fact that the energy needed for the production of primary cells is more than ten times larger than the energy obtainable from those produced, the use of rechargeable cells reduces energy waste. In addition, the development of rechargeable cells can bring various kinds of advantages since primary cells are wasteful, and there are environmental problems associated with their use. To achieve these advantages, research and development of rechargeable cells have been conducted primarily on nickel-cadmium cells, nickel-hydrogen cells and alkaline manganese dioxide-zinc cells. In particular, it is estimated that alkaline rechargeable cells that employ zinc as an anode material have the advantages of high energy density and economically low cost. Furthermore, it is expected that the demand for the alkaline rechargeable cells will increase as the use of toxic lead-acid and nickel-cadmium cells becomes restricted.
However, rechargeable cells employing zinc as an anode material have disadvantages including: instability of the zinc anode material and short cycle life. The main reasons for deteriorating performances of the zinc electrode are uneven dissolution during the discharging period and the Zn deposition during the charging period during repeated charge/discharge cycling. In other words, zinc is dissolved unevenly and irregularly during the discharging period because of minute differences in the surfaces of zinc electrodes. As a result, the zinc electrode surface will become rougher with the progress of the discharge, and the zinc electrode will come to be deformed. On the contrary, during the charging period, zinc deposits in a tree-branch form (which is called a dendrite). The dendrite will grow toward the cathode wit repeated charge/discharge cycling. The deposited zinc will ultimately penetrate the separator to cause an internal short-circuit. Furthermore, with repeated charge and discharge cycling, polarization from equilibrium potential of zinc/zinc ion (II) becomes more severe (increase of overpotential), causing the decomposition of electrolyte, which in turn causes a gas evolution and electrolyte exhaustion. The phenomena such as deformation of zinc anode, dendritic growth, gas evolution and electrolyte exhaustion should be prohibited, because they may extremely threaten the reversibility and stability of not only the zinc anode but also the cell itself.
The effects demanded of the zinc anode additives can be summarized as follows:
i) the prohibition of hydrogen evolution, PA1 ii) the prohibition of the zinc compound's dissolution, PA1 iii) the formation of a rigid zinc electrode surface via electrodeposition together with zinc (the prohibition of dendritic growth), PA1 iv) the formation of a metal surface having good electrical conductivity for the rigid electrodeposition of zinc, PA1 v) the guiding out uniform current distribution, PA1 vi) the improvement of wettability of the zinc electrode, PA1 vii) the improvement of electronic conductivity of the zinc electrode, PA1 viii) the decrease in mass transfer resulting from the formation of a complex with a water soluble zinc compound, PA1 ix) the improvement of availability of the zinc electrode, PA1 x) the maintenance of a porous structure for the zinc electrode.
Taking into account those requirements, methods of using alloys between various kinds of metals and zinc or adding metal oxide have been employed hitherto. In addition, the methods of adding many additives to electrolytes have also been developed (J. Electrochem. Soc., 138, 645 (1991)).
In addition to the above-mentioned problems relating to zinc electrodes, there is a restriction relating to manganese dioxide cathodes caused by the irreversibility of cathodes. For example, in order to obtain 100 or more cycles of charge/discharge performance, the discharge capacity should be restricted not to exceed 25% of one-electron theoretical capacity. This is because the product of one-electron discharge, MnOOH loses its electrochemical activity. The possibility of recharging (oxidizing) manganese dioxide of which discharge capacity is almost utilized (deeply discharged) is limited to the initial few cycles along with a drastic capacity loss. The causes of this irreversibility can be explained in many ways, including the loss of surface conductivity, the increase of internal resistance of the reaction intermediate or product, and the production of Mn.sub.2 O.sub.3, Mn.sub.3 O.sub.4 or ZnO.Mn.sub.2 O.sub.3. According to the McBreen's explanation (Electrochim. Acta, 7, 449 (1962)), the initial crystal lattice is broken down during the first-electron discharging period to produce amorphous MnOOH, which is then reduced to Mn(OH).sub.2 during the second-electron discharging period. This is accumulated in the form of irreversible Mn.sub.3 O.sub.4 or ZnO.Mn.sub.2 O.sub.3 during the charging period to cause the deterioration of the cell performance, which makes charging reaction impossible.
The alkaline manganese dioxide-zinc cells are the most widely used systems in the primary cell market. It is expected that their usage will increase since they not only have reliability but also cause fewer environmental problems, utilize comparatively inexpensive and abundant materials, can easily be discharged at a high rate and show stable discharging performances. They have been found to have, however, critical disadvantages. The charging performance of the alkaline manganese dioxide-zinc cells are not good except in cases that manganese dioxide and electrolyte systems are in a new form, and zinc is not in high purity. In this context, a more detailed description regarding a conventional manganese dioxide-aqueous zinc solution cell system follows.